Passive Intermodulation (PIM) interferences, discussed below, are undesirable but are present in wireless communication systems. FIG. 1 shows a portion of a typical wireless communication system 10 from which PIM may arise. FIG. 1 shows two base stations 12a and 12b, referred to herein collectively as base stations 12. Each base station 12 may serve a plurality of mobile devices 14a and 14b, referred to herein collectively as mobile devices 14, which may include mobile phones, laptop computers, tablet computers, other user equipment (UE) devices, etc. Each base station is in communication with a network 16 which may include the Internet and the Public Switched Telephone Network (PSTN). In a typical wireless communication system there are many base stations and many wireless devices. Each base station 12 may have multiple antennas and multiple transmitters and receivers. Further, more than one base station can be located at the same location, referred to herein as a cellular site.
PIM interferences are a relatively new problem which occurs in crowded spectrum in a wireless communication system. PIM is a form of intermodulation distortion that occurs in passive components such as antennas, cables, connectors or duplexers with two or more high power input signals. PIM can also be generated by rust, corrosion, loose connections, dirt and oxidation. When PIM is present, two or more signals interfere to cause sum and difference signals. Self-generated PIM is caused by a base station's own signals interfering with one another. PIM may also arise when a base station's signals interfere with signals from external sources. Further, PIM may result due to interference of external signals.
The PIM interferences that are observed at cellular sites are usually generated from two or more high power carriers which are mixed together in a non-linear passive component, such as rusty metal, located in the vicinity of the transmit antennas. Those intermodulation products are especially harmful when they land in one of the uplink receive channels since they deteriorate the signal to interference plus noise ratio (SINR) as well as the link budget.
Known solutions to PIM detection such as common public radio interface (CPRI) sniffing and two tones tests require human intervention and external equipment. Moreover, those methods assume that the PIM interferences are (1) only caused by the transmitted signals and they ignore the fact that (2) the intermodulation products could also be created by mixing the own transmitted signals with some signals from the environment, or alternatively, (3) that they could be created by signals from the surrounding environment only.